The present invention relates to optical storage media. Furthermore, this invention relates to reproduction of data from storage media.
Card-type optical storage media have been developed as prepaid cards, credit cards and identification cards, etc., instead of magnetic storage media which are easily forged and the data stored therein are easily rewritten.
Among optical storage media, particularly, holographic storage media are through to be effective for prevention of such illegal usage.
Japanese Unexamined Patent Publication No. 1993(5)-50788 discloses a card-type optical storage medium. Formed on the card are coded diffraction grating patterns. Each pattern formed on a minute area on the card exhibits a specific spatial frequency and diffraction in a specific direction in which light reflected by the pattern is scattered. The diffraction grating patterns are a combination of patterns of the same or different spatial frequencies and directions of diffracting.
In detail, arranged on the card in this unexamined patent publication are diffraction gratings where gratings are arranged in different directions and with different spacing for diffracting incident light in a variety of directions over the card. The card is provided with a pair of diffraction gratings for incident light to be diffracted in a first direction, and another pair of diffraction gratings for incident light to be diffracted in a second direction that is different from the first direction. The two pairs are closely arranged so that directions of diffracted light are detected to reproduce one figure.
The optical card disclosed in Japanese Unexamined Patent Publication No. 1993(5)-50788, however, has drawbacks as follows:
A portion of the diffraction gratings arranged in specific directions, or the minimum unit of the stored data, will not exhibit specific diffraction if physically damaged. This results in drop out of the stored data. The card is thus provided with diffraction gratings arranged in the same direction for high redundancy to prevent such data drop out from occurring. Such arrangement, however, lowers data density of the card.